1. Field of the Invention
The present invention relates to liquid crystal displaying techniques, and in particular to an LED (Light-Emitting Diode) backlight module.
2. The Related Arts
The LED backlight modules that are used in liquid crystal displaying techniques are generally classified in two types, direct backlight modules and side-edge backlight modules. For the side-edge LED backlight modules, with the increasing improvement of LED efficiency, the design for LED backlighting is also evolving, generally from the four-side incidence to two-side incidence, and then single-side incidence. The currently researches and future developments are both directed to the single side incidence.
The side-edge LED backlighting is constructed by arranging LED chips at circumferential edges of a liquid crystal panel and a light guide plate is included so that when the LED backlighting is energized in a sectionized manner, light from the edges of the liquid crystal panel travels through the light guide plate (LGP) to reach a central portion of the liquid crystal panel, providing sufficient backlighting to whole panel by which the liquid crystal panel may display an image. The side-edge LED backlighting generally has two advantages. One is that the number of LED chips used is less and thus the cost is saved. The second is that a thin and light-weighted device body is possible for LED modules are not arranged on the back side of the liquid crystal panel and are instead located on the sides. This reduces the overall thickness of the liquid crystal panel and thins a device body.
Referring to FIG. 1, a schematic view is given to illustrate the operation principle of a conventional side-edge LED backlight module. Single-side incidence is taken as example. The conventional side-edge LED backlight module generally comprises a light guide plate 10, an LED light bar 11, a backplane 12, and a heat sink 13. The light guide plate 10 is disposed on the backplane 12 and the heat sink 13 is arranged between the light guide plate 10 and the backplane 12. The LED light bar 11 that serves as a light source is arranged at one side of the light guide plate 10. The LED light bar 11, when put into operation, generates heat that is dissipated through the heat sink 13. A mold frame 14 is set above edges of the light guide plate 10 and a reflector board 15. The reflector board 15 is located above a light incident site between the LED light bar 11 and the light guide plate 10 to make the light emitting from the LED light bar 11 entering the light guide plate 10 as much as possible. The light is transmitted through the light guide plate 10 in the direction indicated by arrowheaded lines. Phantom line circles show where light is subjected to total internal reflection. By means of total internal reflection, light that enters from a side of the light guide plate 10 can be guided to travel along the light guide plate 10. The light guide plate 10 comprises grid dots 16 formed on an undersurface thereof and optic films 17 arranged above a top surface thereof. The grid dots 16 function to destroy the total internal reflection in order to allow light to emerge from the top surface of the light guide plate 10 to illuminate a liquid crystal panel (not shown) that is located above the light guide plate 10. The heat sink 13 is generally an aluminum extrusion. The backplane 12 can be an aluminum backplane. In the side-edge backlight module, the spacing distance between the LEDs and the light guide plate directly affects the efficiency that the LED is coupled to the light guide plate. The smaller the distance is, the higher the coupling efficiency will be. Namely, the backlighting efficiency is enhanced.
Referring to FIGS. 2 and 3, a perspective view is given to show the conventional side-edge LED backlight module with the light guide plate removed and FIG. 3 is a cross-sectional view of FIG. 2 showing the site where a bolt is located. It is often that an LED light bar 21 is fixed on a heat sink 22. The heat sink 22 is fixed to a backplane 24 by bolts 23 to fix the relative position thereof with respect to the backplane 24. To simplify the drawings, hatching is only shown in the site of the bolt 23 in the drawings. The LED light bar 21 comprises a plurality of LED chips 211 to serve as a light source. The backplane 24 is extended upward to form LGP positioning columns 25.
As shown in FIG. 4, which is a perspective view showing the backlight module of FIG. 2 with a light guide plate mounted thereto, the light guide plate 26 is fixed by the LGP positioning columns 25 provided on the backplane 24. The light guide plate 26 forms notches corresponding to the LGP positioning columns 25, whereby the LGP positioning columns 25 are allowed to extend upward through the notches to fix the light guide plate 26.
As shown in FIG. 5, a cross-sectional view is given to illustrate expansion of the light guide plate of the backlight module shown in FIG. 2. When the environment in which the backlight module operates changes, especially when the light guide plate 26 is subjected to expansion caused by being heated or absorbing humidity, it is likely that the light guide plate gets into contact with a short edge of the LED light bar 21. Such a variation may cause two risks, one being the risk of LED reliability, where the LEDs may get hit and thus damaged by the light guide plate and the other being waving occurring on vertical surface of the light guide plate due to the light guide plate being fixed in position by the LGP positioning columns and not allowed to extension in a lengthwise direction.
The known solutions keep a sufficient clearance for movement of the light guide plate in order to prevent waving of the light guide plate caused by the expansion of the light guide plate resulting from being heated or absorbing humidity, whereby the spacing distance between the light guide plate and the LEDs is increased and this reduces light coupling distance and thus greatly affects the efficiency of backlighting.